In the Bluetooth Special Interest Group (SIG), a new feature, Bluetooth Low Energy (BLE) Long Range (BLR), is being standardized. Forward error correction (FEC) coding, spreading, and/or coherent demodulation are being introduced to the Bluetooth standard for receiver implementation to improve receiver sensitivity and range. To fully take advantage of these improvements, it is important that the receiver has good synchronization performance. Without good synchronization performance, the performance may be limited by synchronization error.
Achieving synchronization at low signal-to-noise ratio (SNR) is challenging in BLE. This is because the radio requirements are very relaxed, and the initial frequency offset at the transmitter may be as large as 50 ppm. This translates to 150 kHz frequency offset at the transmitter. In combination, the frequency offset at the transmitter and receiver can result in a very large initial frequency error. Furthermore, a frequency drift up to +/−50 kHz may add to the initial frequency error. Many technologies that adopt low-cost radio front ends having relaxed accuracy requirements face similar challenges when they extend the range to work at low SNR.
Existing approaches to accommodate a large initial frequency error perform initial time synchronization in a receiver based on differential detection, which may be implemented using a phase discriminator. These approaches may have certain deficiencies. For example, such a time synchronizer has limited sensitivity, and cannot operate at low SNR. This is especially true in the region where noise power is stronger than the desired signal power, i.e., where SNR is less than 0 dB. Thus, there is a need for a method of acquiring synchronization at low SNR in the presence of large initial frequency error.